This invention relates to magnetic tape transports, sometimes referred to as tape units, tape drives, tape decks, etc., for use with a replaceable tape assembly such as that known as a tape cassette or cartridge, and more particularly to digital magnetic tape transports used as a subsystem to enable a host system to obtain access to data on the magnetic tape. Still more particularly, the invention pertains, in such tape transports, to a system for controlling the tension of the tape, especially when it is at a standstill.
Cassette tape transports, particularly to those operating in streaming mode, as contrasted with start/stop mode, have come to find extensive use as peripherals of computer systems. U.S. Pat. No. 4,163,532, filed by Sakai and assigned to the assignee of the instant application, discloses one such streaming cassette tape transport, or streamer. This prior art streamer comprises a pair of reel motors, to be drivingly coupled one to each reel of a tape cassette, for bidirectionally transporting the tape between the two reels under the direction of a motor control circuit forming a part of a tape speed control servo. Also included in the tape speed control servo is a tape speed sensor comprising a roll for frictional engagement with the tape, and an encoder for generating a series of pulses representative of the actual tape speed. The motor control circuit causes one of the reel motors to be driven in response to the tape speed sensor output pulses for constant speed tape transportation in each direction.
The tape must of course travel under proper, constant tension between the reels in order to enable the transducer to correctly write or read data on the tape. To this end the motor control circuit additionally comprises tension control means which afford constant tape tension in the face of varying diameters of tape rolls on both reels. The tape must be held under tension when it is not only traveling but also at rest, in order to prevent the tape from slackening due to external forces.
Conventionally, the pair of reel motors were both energized in opposite directions for holding the tape under tension when it is at rest. The sum of the magnitudes of currents flowing through both reel motors during such times was kept the same regardless of variable tape diameters on both reels, so that a current of greater magnitude flowed through whichever of the reel motors associated with the reel carrying a greater amount of tape. This conventional scheme was effective in the sense that tape tension could be controlled according to tape roll diameters on both reels, though to a limited degree.
When too much difference existed between the tape roll diameters, as in tape positions near the beginning and end of the tape, the prior art system was not necessarily capable of causing the reel motors to be energized accordingly. The tape tended to travel slowly in the worst case because of imbalance in the magnitudes of currents flowing through both reel motors, especially when the tape was exceptionally light weight.